USB: add pure specifiers and emit vtable

[ardnew]

Issue #130 correctly identifies a newly-added method as pure virtual and fixes the declaration. However, for some reason it didn't address all of the other virtual methods in that same class (PluggableUSBModule) that do not define a default implementation.

The only virtual method that has a default implementation is provided inline in the class interface:

    virtual void callback_reset() {};

These issues combined prevent the compiler from being able to emit a vtable for the PluggableUSBModule class, thus preventing users from correctly subclassing it or any one of its derived classes such as USBCDC, USBHID, USBMIDI, etc. Refer to the following answer on StackOverflow for a detailed explanation of the issue:

https://stackoverflow.com/a/57504289/1054397

This PR adds the pure specifier (= 0) to all of the virtual methods in this class that do not have a default implementation. It also moves the default empty definition of virtual void callback_reset() to the class definition in USB/PluggableUSBDevice.cpp so that this class complies completely with the criteria for emitting a vtable.

Note

The error that I was encountering prior to these changes was pretty
cryptic (from PlatformIO):

.pio/build/hid/src/target.cpp.o: In function `foo()':
USBHID/src/PluggableUSBHID.h:53: undefined reference to
    `vtable for arduino::internal::PluggableUSBModule'
.pio/build/hid/src/target.cpp.o: In function `foo()':
foo.hpp:100: undefined reference to
    `vtable for arduino::internal::PluggableUSBModule'
collect2: error: ld returned 1 exit status
*** [.pio/build/hid/firmware.elf] Error 1

Even stranger, the error would only be generated with a debug build;
i.e., the only difference in command-line arguments was the additional
CFLAGS of -Og -g2 -ggdb2. Without the debug flags, my project was
building without error.

With the changes in this PR, my project now builds with and without
these additional debug flags. Further verification was performed by
testing the example sketches Keyboard, KeyboardRaw, and Mouse
from library USBHID as well as using the core Serial object for
ordinary USB serial I/O (USBCDC).

[CLAassistant]

All committers have signed the CLA.

[facchinm]

LGTM! Thanks!